The Squirrel-Cage Rotor

If the single loop of wire of Fig. 18-1 be replaced by several similar loops uniformly displaced circumferentially about the same axis, as in Fig. 18-2, the main field would never be without immersed bars and the turning effort on the whole would be less pulsating than with the single loop.

Fig. 18-2. Step in Development of Squirrel-Cage Rotor

Fig. 18-3. Winding on Squirrel-Cage Rotor

In industrial induction motors many bars are used and, instead of being connected in loops as in Fig. 18-2, they are connected to end rings as in Fig. 18-3. A rotor with such an electrical construction is called a squirrel-cage rotor. The squirrel cage itself is built up of copper or brass bars welded to end rings of similar material, or is cast of aluminum alloy as one piece in place without insulation on a laminated steel cylindrical core. A finished rotor of squirrel-cage construction is
shown in Fig. 18-4. The extreme simplicity and ruggedness of construction and the freedom from moving electrical contacts makes the squirrel-cage motor the most popular in general industrial use.